1. Field of the Invention
The present invention relates to a control apparatus for a continuously variable transmission that is capable of selecting an automatic shift mode in which a transmission gear ratio is automatically changed according to the driving condition of a vehicle or a manual shift mode in which the transmission gear ratio is set according to a shift operation performed by a driver.
2. Description of the Related Art
Continuously variable transmissions (CVTs) such as belt CVTs and toroidal CVTs are capable of steplessly setting an optimum transmission gear ratio according to driving conditions such as throttle opening, vehicle speed, and engine rotational speed. However, if a driver wishes to obtain a large drive torque when driving up a hill or during acceleration or if the driver wishes to use a large amount of engine braking when driving down a hill, due to a shift pattern being set in advance for each driving area, it is difficult to set other transmission gear ratios, thus causing comfort problems with insufficient drive torque or insufficient deceleration for the driver.
To address such problems, continuously variable transmissions having a manual shift mode in which a driver can set a transmission gear ratio as desired have been proposed in recent years. In such continuously variable transmissions having the manual shift mode, a plurality of pseudo-transmission gear stages for controlling the transmission gear ratio to a certain fixed transmission gear ratio are generally set. When the shift mode is changed from the automatic shift mode to the manual shift mode, the transmission gear stage is sequentially shifted to the low-speed side each time the driver turns on a downshift switch, and the transmission gear is sequentially shifted to the high-speed side each time the driver turns on an upshift switch.
The continuously variable transmissions described above generally take measures against, for example, torque shock by setting a certain limit (guard value) on the amount of change in the transmission gear ratio upon shifting and by preventing sudden discharge or absorption of inertia torque caused by rapid changes in the transmission gear ratio (see, for example, Japanese Unexamined Patent Application Publication No. 2001-330125).
In the continuously variable transmissions having the manual shift mode, because of the control of the continuously variable transmissions, a target transmission gear ratio may be changed significantly at certain timing such as immediately after the driver has switched the shift mode from the manual shift mode to the automatic shift mode. Specifically, this situation occurs, for example, when the driver releases a acceleration pedal and switches the shift mode to the automatic shift mode from the manual shift mode where the vehicle is traveling at a high speed with a low-speed transmission gear stage. If the guard value is active in the shift operation that involves a large change in the target transmission gear ratio, the time for shifting is increased and a change occurs with unnatural behavior in which the engine speed slows down. In the continuously variable transmissions, furthermore, shifting is performed continuously, and therefore it is difficult to give a feeling like a stepped transmission to the driver, whereby the driver may feel uncomfortable.
If the guard value for the amount of change in the transmission gear ratio is eased, on the other hand, the time for shifting can be reduced, whereas, large inertia torque is generated at once when the shifting operation is completed. Therefore, it is difficult to sufficiently reduce torque shock.